The present invention relates to hybrid bearings, and in particular to hybrid bearings used in conjunction with hard disk drive spindle motors. The invention further relates to hybrid bearings used in disk drive spindle motors having an air radial bearing and a fluid thrust bearing.
Disk drive memory systems store digital information on magnetic disks. The information is stored on the disks in concentric tracks divided into sectors. The disks themselves are mounted on a hub, which rotates relative to the disk drive enclosure. Information is accessed by means of read/write heads mounted on pivoting arms able to move radially over the surface of the disks. This radial movement of the transducer heads allows different tracks to be accessed. Rotation of the disks allows the read/write head to access different sectors on the disks.
In operation, the disk or disks comprising the magnetic media are rotated at very high speeds by means of an electric motor generally, but not necessarily, located inside the hub that supports the individual disks. Bearings mounted inside the hub allow the hub to rotate about a fixed shaft. Alternatively, the hub is fixed to a rotating shaft carried by bearings mounted to the base or enclosure of the disk drive. In either configuration, the bearings are typically ball-bearings or fluid bearings. Bearings having a fluid lubricant are desirable for disk drive applications because of their inherently low nonrepeatable runout and low acoustical noise. However, these bearings suffer from several shortcomings. For instance, oil filled bearing designs have been difficult to seal. In particular, where oil filled bearings are used to support a rotatable hub in radial and axial directions, it is extremely difficult to balance the pressure exerted on the oil between all of the surfaces of the bearing. As a result, oil can be forced from between the bearing surfaces, contaminating the interior of the disk drive. Such contamination may cause a failure or a decreased performance of the drive. Bearing systems incorporating an oil lubricant also have a limited maximum rotational speed, due to large power consumption at high speeds.
Alternative bearing designs have utilized air as the lubricant with bearings having grooved bearing surfaces to generate areas of increased pressure when the surfaces of the bearing move in opposition to each other. However, such designs typically only have a unidirectional thrust mechanism, and therefore the disk drive can only be operated when the device is in certain orientations (e.g. upright) or the device cannot withstand shock in certain directions (e.g. the axial direction). Furthermore, air bearing designs have typically featured a relatively small diameter radial bearing surface, resulting in bearings that have inadequate stiffness. Adequate stiffness is difficult to achieve in an air bearing because air has a viscosity that is much lower than the viscosity of oil or other conventional liquid lubricants. Therefore, conventional air bearing designs result in a bearing that cannot maintain the rotating components in a precise relationship to the stationary components when bearings constructed in accordance with those designs are subjected to external forces. Stated differently, a bearing that lacks stiffness will allow the rotating disks to deviate from the desired alignment when the drive is subjected to external forces.
Air is desirable as a bearing lubricant because its use removes concerns about leakage and outgassing resulting from the presence of oil. In addition, the viscosity of air varies less with changes in temperature compared to the viscosity of oil or other lubricants. Furthermore, air bearings generate less acoustical noise and less nonrepeatable runout than ball-bearing designs, and consume less power in comparable situations due to decreased friction compared to oil filled bearings. However, existing air bearing designs use extremely high rotational speeds and/or extremely tight internal clearances to increase the stiffness of the bearing in order to achieve stiffness levels that are comparable to the stiffness of oil-filled bearings. This is due in large part to the fact that the viscosity of air is approximately {fraction (1/700)}th the viscosity of oil. However, increased rotational speeds generally reduce the storage capacity of the disk drive because of limitations in read/write channel data rates. Also, the tight internal clearances typically employed by known air bearing designs increase manufacturing costs.
It would be desirable to provide a bearing system for a disk drive motor assembly that presents a low risk of contamination to the storage media, and one that is stiff enough to support a heavy disk pack and/or withstand external shocks. In addition, it would be desirable to provide such a device with reduced friction, reduced power consumption and wear and tear, and a longer life than conventional bearing systems. Furthermore, it would be desirable that such a device be easy to manufacture in large volumes and at low cost.
In accordance with the present invention, an apparatus and method for supporting a rotating component is disclosed. In particular, the invention provides a computer disk drive spindle bearing having an air-filled bearing for support in a radial direction and an oil-filled bearing for support in an axial direction. In a preferred embodiment, the air bearing has a large diameter with respect to the spindle. In addition, the present invention provides an oil-filled bearing that is resistant to leakage for supporting the rotating hub in an axial direction.
The device generally includes a stationary shaft fixed or interconnected to a base. Affixed to the stationary shaft is a thrust plate for use as a part of an oil filled bearing. The thrust plate cooperates with upper and lower bearing plates interconnected to a hub to support the hub in an axial direction. The space between the thrust plate and the upper and lower bearing plates is filled with a viscous oil. The viscous oil insures that the bearing has high stiffness and high load capacity.
Also interconnected to the stationary shaft is an air bearing element having a large diameter. A large diameter is desirable because the stiffness of such a bearing increases with the cube of the diameter. In a preferred embodiment, the air bearing element substantially fills the volume defined by the interior surface of the rotating hub. In a more preferred embodiment, the diameter of the air bearing is greater than or equal to the diameter of the thrust plate of the oil filled bearing. In another preferred embodiment, the volume of the air bearing element is at least about 80% of the volume defined by the interior surface of the rotating hub.
The air bearing is also preferably relatively tall. As with the relatively large diameter, a relatively tall bearing is desirable because it provides greater stiffness and greater load capacity. Furthermore, increases in stiffness and load capacity gained by making the bearing larger can allow the bearing to be manufactured to tolerances that are no more stringent than those typically maintained for oil-filled bearing disk drive designs. In a preferred embodiment, the length of the air bearing is at least about 50% of the height of the hub surface about which storage media disks are stacked. In a more preferred embodiment, the air bearing extends from the center line of the rotor of the electric motor used to rotate the hub about the spindle, to a level at least about halfway along the hub between the hub flange and the disk clamp.
As discussed above, the relatively large size of the air bearing element increases the stiffness of the air bearing. In addition, the air bearing element may be provided with pressure generating ridges or grooves. These ridges or grooves direct air away from the edges of the bearing to increase the air pressure along the surface of the bearing, improving the stiffness of the bearing. Increased stiffness allows the bearing to support a heavier disk pack, and to avoid damage that might otherwise be sustained due to external forces or shocks.
The bearing of the present invention also provides a disk drive spindle and housing that are themselves very stiff. This is due to the use of a stationary shaft, which can be affixed to the enclosure at both ends to ensure a very stiff disk drive assembly. The increased stiffness of the design improves the accuracy of the read/write head in relation to the storage disks, and lessens low frequency vibrations.
The unique combination of an oil-filled bearing for supporting the rotatable hub in an axial direction and an air-filled bearing for supporting the rotatable hub in a radial direction provides a bearing apparatus having lower friction than an all oil-filled bearing design. Therefore, the bearing of the present invention consumes less power in operation and permits the disks to rotate at higher speeds than an all oil-filled bearing design having similar load bearing capabilities. In addition, the bearing of the present invention has greater load capacity than an air bearing of similar dimensions, because an air bearing must be extremely large to provide comparable support in an axial direction. Therefore, the present design offers a bearing with a higher load capacity and/or smaller size than an all air bearing design. Yet another advantage of the bearing of the present invention is the low nonrepeatable runout obtained by using fluid filled bearings exclusively. Therefore, runout problems associated with ball bearing or contact bearing designs are avoided.
Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings.